Bump netty4.version from 4.1.115.Final to 4.2.1.Final #5498

Job	Run time
build-all	3m 28s
tests (es, -Pelasticsearch6, es6-java8, 8)	28m 28s
tests (es, -Pelasticsearch7, es7-java8, 8)	29m 28s
tests (es, -Pelasticsearch8, es8-java8, 8)	30m 36s
tests (es, -Pjava-11, -Pelasticsearch60, es60, 11)	29m 39s
tests (es, -Pjava-11, -Pelasticsearch7, es7, 11)	30m 31s
tests (es, -Pjava-11, -Pelasticsearch6, es6, 11)	29m 55s
tests (es, -Pelasticsearch60, es60-java8, 8)	28m 24s
tests (es, -Pjava-11, -Pelasticsearch8, es8, 11)	31m 20s
	4h 1m 49s

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